A biosensor is used to detect or quantify a target substance, namely an antigen, contained in a sample. A high affinity between an antigen and an antibody and a high affinity between Protein A and an antibody may be used in the biosensor. Specifically, Protein A is immobilized on the biosensor. Furthermore, an antibody is immobilized on the Protein A. When the antigen is supplied to the biosensor, the antigen is immobilized on the biosensor due to the high affinity between the antigen and the antibody.
Patent Document 1 discloses a prior biosensor utilizing the high affinity between Protein A and biotin. FIG. 2 shows a biosensor disclosed in FIG. 7 of the Patent Document 1.
According to the description regarding FIG. 7 of the Patent Document 1, the biosensor is used for screening an activity of a living molecule. The biosensor comprises a monolayer 7, an affinity tag 8, an adaptor molecule 9, and a protein 10. The monolayer 7 is composed of a self-assembled monolayer represented by chemical formula: X—R—Y (see Page 24 lines 23-26, Page 25 lines 3-20, Page 25 line 27-Page 26 line 13, and Page 26 lines 14-22 of Patent Document 1). Examples of X, R, and Y are HS—, an alkane, and a carboxyl group, respectively (see Page 25 lines 3-20, Page 25 line 27-Page 26 line 13, and Page 28 lines 21-23 of Patent Document 1).